A car seat apparatus configured such that when a vehicle turns, a seat back plate portion is swiveled to orient toward a turning direction so as not to allow the feature of holding an occupant to be impaired due to a lateral acceleration produced in a direction reverse to the turning direction is hitherto known in the art (e.g., Patent Document 1). According to the invention disclosed in the Patent Document 1, a control exercised is such that the lateral acceleration to be imposed on the car is estimated by computation, and when the lateral acceleration exceeds a predetermined threshold value, the orientation of the seat back plate portion is changed (such a control will be hereinafter referred to as “seat posture control”).